Traditional pulse oximetry has been recommended as a standard of care for nearly all areas of the medical care. By measuring the relative absorptions of different wavelengths of light, transilluminating a given region of the body (normally the fingertip or the earlobe), important systemic and physiological information related to the cardiovascular system can be measured. Currently, arterial blood flow is used as the main source of information because it provides a universal indication of the amount of oxygen available within circulating blood. Additionally, because of the pulsatile nature of the arteries, it is relatively easy to separate from other background absorption, as described in detail by Tremper et al., “Pulse Oximetry,” Anesthesiology, vol. 70, (1989), pp. 98-108, incorporated herein by reference. However, under many circumstances, a cellular oxygen consumption provides a better measure of metabolic health, as discussed by Guyton et al., “Textbook of Medical Physiology,” 9th ed., (W. B. Saunders Company, 1996), incorporated herein by reference.
It is well known that there is a critical balance of between the supply and demand for oxygen delivery within the body. In fact, global tissue hypoxia (severe oxygen deficiency) has been found to be an important indicator of serious illnesses, leading up to multi-organ failure and death. The level of tissue hypoxia provides information directly related to the “golden hours” of health care. These are the hours when recognition and treatment of the condition will provide maximal outcome benefit towards the patient's recovery. Until recently, however, there has not been a reliable measurement, or group of measurements for this indicator.
Clinical results suggest that a goal-directed therapy that utilizes early measurements of central venous oxygen saturation during resuscitation “ . . . has significant short-term and long-term benefits . . . ” (Rivers et al., “Early Goal-Directed Therapy in the Treatment of Severe Sepsis and Septic Shock,” The New England Journal of Medicine, 345 (2001), pp. 1368-77). These benefits are believed to be the result of an earlier identification of the patients who are at a high risk for eventual cardiovascular collapse. By following a goal-directed therapy that includes the use of central venous oxygen saturation, researchers found that patient mortality rates related to shock could be reduced from 46% down to just over 30%.
Other studies have revealed that venous measurements can provide useful information about the function of the left heart as well as changes in cardiac output. Additionally, early localized, peripheral venous monitoring may be an extremely useful early indicator of internal systemic problems. For example, in low blood flow conditions, significant decreases in peripheral venous oxygen saturation will occur due to the fact that the slower blood flow provides local tissue with more time to extract available oxygen. The increased amount of extraction will be directly reflected by the decreases in the peripheral venous saturation.
One method for measuring oxygen consumption consists of measuring the difference between arterial and venous oxygen saturations. Arterial measurements can easily be acquired because of their natural volumetric pulsations. These volumetric pulsations make it possible to optically differentiate arteries from the non-pulsatile background, as depicted in FIG. 1. Moreover, arterial volumetric pulsations are the key to modern pulse oximetry methods. Consequently, the arterial side of oxygen consumption measurements is fairly simple.
FIG. 1 depicts the optical absorption of body tissue. Optical absorption is plotted along the vertical axis, while the horizontal axis depicts time. The lower segment 10 represents absorption due to bone, skin, and other non-blood-bearing tissue. Segment 12 depicts absorption due to venous and capillary blood, while segment 14 represents the component of arterial blood flow that is non-pulsatile. Segments 10, 12, and 14, together, comprise a stationary and substantially novarying (‘DC’) component of the optical absorption of tissue. Segment 16 of the absorption is due to the pulsation of arterial blood volume. Traditional pulse oximeters measure the oxygen saturation at the artery by using an empirical formula relating arterial oxygen saturation (SaO2) to pulsatile photo-plethysmographic (PPG) signals at two wavelengths. In the prior art method, one or more light sources, such as LEDs, provide illumination, typically monochromatic, at one or more wavelengths. Photons from the LEDs pass through the skin. Although the photons illuminate in all directions, the average light path travels through a portion of the tissue and then back to a photodetector. Light detected at the photodetector has periodic (AC) and constant (DC) components, with the constant component primarily governed by light source intensity, ambient light, detector sensitivity, soft tissue, bone, venous blood, capillary blood, and non-pulsatile arterial blood. The AC component, on the other hand, captures the pulsating arterial blood.
Arterial blood flow is measured using LEDs emitting at the wavelengths specified with respect to the isobestic point of hemoglobin and oxygenated hemoglobin, at approximately 800 nanometers. At the isobestic wavelength λi, the optical absorption is insensitive to the fraction of oxygenated hemoglobin. ‘RED’ refers to a measurement performed at a wavelength shortward of λi, while ‘IR’ refers to a measurement performed at an infrared wavelength longward of λi.
The light transmitted through a tissue path including an arterial volume is monitored at each of the two (RED and IR) wavelengths, and the ratio,   R  =                    ln        ⁡                  (                                                    I                out                            ⁡                              (                systole                )                                                                    I                out                            ⁡                              (                diastole                )                                              )                    RED                      ln        ⁡                  (                                                    I                out                            ⁡                              (                systole                )                                                                    I                out                            ⁡                              (                diastole                )                                              )                    IR      is taken as a measure of arterial oxygen saturation.
The pulsatile nature of the arterial signal differentiates signals attributed to the arterial blood from the one due to the venous blood and other surrounding tissue. The vein, however, does not pulsate, hence the standard oxygen saturation algorithm does not apply to the venous O2 measurement. The difficulty of noninvasively measuring local venous saturation means that clinically acceptable oxygen consumption measurements are presently made through the use of blood drawn from highly invasive catheters. Unfortunately, since this type of measurement is both invasive and discrete (i.e., discontinuous) it is ordered mainly for critical care patients.
Current technologies for determination of venous saturation are extremely invasive and only provide information on the mixed venous saturation (a more global value) which is less sensitive to a low flow condition than a local venous saturation measurement. It is also desirable to provide a noninvasive technology that allows sensors to be used readily all over a hospital (emergency room, outpatient, etc.).